Rhodium(III)‐Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3‐Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization

Metal allyl species are versatile intermediates in a number of coupling reactions. However, Rh(III) π-allyls have been rarely elaborated in the context of asymmetric C-H functionalization. Reported herein is chiral Rh(III) cyclopentadienyl-catalyzed enantioselective synthesis of lactams and isochromenes via oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3-enynes. The reaction proceeds via a C-H activation, alkenyl to allyl rearrangement, and nucleophilic cyclization cascade. The mechanistic details of the [4+1] annulation of N-methoxybenzamide and an 1,3-enyne have been elucidated by a combination of experimental and computational methods. DFT studies indicate that following the C-H activation and alkyne insertion, a Rh(III) alkenyl intermediate undergoes δ-hydrogen elimination of the allylic C-H via a six-membered ring transition state to produce a Rh(III) enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare Rh(III) η4 ene-allyl species with π-agostic interaction undergoes SN2'-type external attack by the nitrogen nucleophile, instead of C-N reductive elimination, as the stereodetermining step.